Whatever its origin, groundwater spends a long time in an aquifer. It has time to dissolve minute quantities of minerals which can give it definite characteristics such as hardness or taste. It is often bottled and sold as mineral water. Groundwater is useful in many ways. It sustains the flow of rivers from which we take water for drinking and many industrial uses. And by sinking a well or borehole into an aquifer, groundwater can be pumped to the surface. The borehole can often be sited precisely where the water is needed-in a village or factory compound, for example- so avoiding the need for expensive water mains.

Rather than operate a borehole pump whenever water is needed, the usual arrangement is to pump water from the borehole at regular intervals to a tank or distribution reservoir on high ground or on top of a water tower. From here the water can flow by gravity to wherever it is needed. A distribution reservoir holds enough water to supply the area it serves for about a day.

Pumping water from an aquifer lowers the water table. This may reduce or stop the natural overflow from the aquifer, causing springs or small streams to dry up. At some places pumping from wells reduces the natural outflow. Some streams on chalk flow less than they used to do; in part this is the result of abstraction. People must decide which they value most-a cheap supply of water or preserving the countryside exactly as it was.

But some changes are blamed unfairly on abstraction. Earlier when there was still little pumping from the Chalk aquifer, water levels were low; low rainfall was the cause then. Although pumping out water can bring problems if aquifers do not have a chance to recharge, groundwater is such an enormous resource that it would be unthinkable to stop using it. But there is the cost of alternatives. On cannot simply build, nor afford to build enough reservoirs to replace the aquifers. Reservoirs provide a relatively small supply of water and they are expensive to build, but relatively easy to understand. Aquifers hold enormous amount of water but money has to be raised t build them up. In the light of increasing concern about the global warming it is better to have more understanding of water

An Aquifer often lies on top of a layer of less permeable rock. This stops or slows the downward flow of groundwater and helps to separate the water in this aquifer from flow in other aquifers that may be underneath it. And unlike water in rivers or streams, groundwater is under hydrostatic pressure that is greater than atmospheric pressure. Thus it can flow upwards, as well as downwards or sideways, just like water in the plumbing system of a house.

This is especially important when an aquifer dips beneath another layer that is much less permeable. As usual, rainwater fills up the aquifer and water flows out from it where it meets the surface. Below the impermeable layer, water is trapped in the aquifer, which is said to be confined. The water presses on the less-permeable confining layers above and below it.

If one drills a borehole into a confined aquifer, water will rise up the borehole until the column of water is enough to balance the pressure in the aquifer. Most major aquifers have a confined portion, where the aquifer is covered by impermeable rock layers and an unconfined portion, where the aquifer covered simply by soil or river or glacial deposits. Rainwater enters the aquifer through the unconfined part.

If many boreholes are drilled into the aquifer and found the level of water in all of them, it could be imagined a surface made by joining all the individual levels. In the unconfined part of the aquifer this surface would be the water table. It separates the saturated part of the aquifer from the unsaturated zone above it. But in the confined part of the aquifer, the spaces between mineral grains, every pore in fact, is filled with water: there is no unsaturated zone. Here the surface is an imaginary one called the pressure surface, or potentiometric surface. It passes through the confining layer somewhere above the aquifer.

If the unconfined part of the aquifer is beneath high ground, and the confined part beneath low ground, then the potentiometric surface may be above ground level. If a borehole is drilled into the aquifer the groundwater will be under sufficient pressure to overflow from the borehole. Such a borehole is called artesian well. But the borehole must go deep enough to reach the aquifer, even if this means drilling far below the potentiometric surface. Otherwise it will yield little or no water from the impermeable confining layer.

In many parts of world, the availability of water form artesian wells makes agriculture possible.